jetCheck: when property fails, report useful things closer to the top of the stack trace

jetCheck/src/jetCheck/PropertyFalsified.java

@@ -9,41 +9,81 @@ import java.util.function.Supplier;
  */
 @SuppressWarnings("ExceptionClassNameDoesntEndWithException")
 public class PropertyFalsified extends RuntimeException {
-  static final String FAILURE_REASON_HAS_CHANGED_DURING_MINIMIZATION = "!!! FAILURE REASON HAS CHANGED DURING MINIMIZATION !!!";
+  static final String FAILURE_REASON_HAS_CHANGED_DURING_MINIMIZATION = "Failure reason has changed during minimization, see initial failing example below";
   private static final String SEPARATOR = "\n==========================\n";
   private final PropertyFailureImpl<?> failure;
   private final Supplier<DataStructure> data;
+  private final String message;
 
   PropertyFalsified(PropertyFailureImpl<?> failure, Supplier<DataStructure> data) {
     super(failure.getMinimalCounterexample().getExceptionCause());
     this.failure = failure;
     this.data = data;
+    this.message = calcMessage();
   }
 
   @Override
   public String getMessage() {
-    String msg = "Falsified on " + failure.getMinimalCounterexample().getExampleValue() + "\n" +
+    return message;
+  }
+
+  private String calcMessage() {
+    StringBuilder traceBuilder = new StringBuilder();
+    
+    String msg = "Falsified on " + valueToString(failure.getMinimalCounterexample(), traceBuilder) + "\n" +
                  getMinimizationStats() +
                  failure.iteration.printToReproduce();
 
+    Throwable failureReason = failure.getMinimalCounterexample().getExceptionCause();
+    if (failureReason != null) {
+      Throwable rootCause = getRootCause(failureReason);
+      appendTrace(traceBuilder, 
+                  rootCause == failureReason ? "Property failure reason: " : "Property failure reason, innermost exception (see full trace below): ", 
+                  rootCause);
+    }
+
     if (failure.getStoppingReason() != null) {
-      msg += "\n Shrinking stopped because of " + StatusNotifier.printStackTrace(failure.getStoppingReason());
-      msg += SEPARATOR;
+      msg += "\n Minimization stopped prematurely, see the reason below.";
+      appendTrace(traceBuilder, "An unexpected exception happened during minimization: ", failure.getStoppingReason());
     }
     
     Throwable first = failure.getFirstCounterExample().getExceptionCause();
     if (exceptionsDiffer(first, failure.getMinimalCounterexample().getExceptionCause())) {
       msg += "\n " + FAILURE_REASON_HAS_CHANGED_DURING_MINIMIZATION;
-      msg += "\n Initial value: " + failure.getFirstCounterExample().getExampleValue() + "\n";
-      if (first != null) {
-        msg += "\n Initial exception: " + StatusNotifier.printStackTrace(first) + SEPARATOR;
+      StringBuilder secondaryTrace = new StringBuilder();
+      traceBuilder.append("\n Initial value: ").append(valueToString(failure.getFirstCounterExample(), secondaryTrace));
+      if (first == null) {
+        traceBuilder.append("\n Initially property was falsified without exceptions");
+        traceBuilder.append(secondaryTrace);
       } else {
-        msg += "\n Initially property was falsified without exceptions\n";
+        traceBuilder.append(secondaryTrace);
+        appendTrace(traceBuilder, "Initially failed because of ", first);
       }
     }
-    return msg;
+    return msg + traceBuilder;
   }
 
+  private static Throwable getRootCause(Throwable t) {
+    while (t.getCause() != null) {
+      t = t.getCause();
+    }
+    return t;
+  }
+
+  private static void appendTrace(StringBuilder traceBuilder, String prefix, Throwable e) {
+    traceBuilder.append("\n ").append(prefix).append(StatusNotifier.printStackTrace(e)).append(SEPARATOR);
+  }
+
+  private static String valueToString(CounterExampleImpl<?> example, StringBuilder traceBuilder) {
+    try {
+      return String.valueOf(example.getExampleValue());
+    }
+    catch (Throwable e) {
+      appendTrace(traceBuilder, "Exception during toString evaluation: ", e);
+      return "<can't evaluate toString(), see exception below>";
+    }
+  }
+  
   private String getMinimizationStats() {
     int exampleCount = failure.getTotalMinimizationExampleCount();
     if (exampleCount == 0) return "";